Method of making glass-ceramics by devitrification in a single stage



United States Patent 3,352,656 METHOD OF MAKING GLASS-CERAMICS BY DEVITRIFICATION IN A SINGLE STAGE Peter William McMillan and Graham Partridge, Stafiord, England, assignors to The English Electric Company Limited, London, England, a British company No Drawing. Filed May 16, 1966, Ser. No. 550,182 4 Claims. (CI. 65-33) This invention relates to devitrified glass-ceramics, and is a continuation-in-part of United States application Ser. No. 269,333, filed Apr. 1, 1963, now abandoned.

Hitherto, in the process of controlled devitrification it has been considered necessary first to heat the glass to a nucleation temperature, at which temperature the glass was maintained for a time dependent on its composition, and then to heat the glass further to a higher temperature at which devitrification took place.

We have discovered that with many compositions of glass it is unnecessary to heat the glass in two stages in this manner.

According to the present invention, therefore, a method of making a glass-ceramic article includes forming the article in a glass which is capable of controlled devitrification and contains a nucleating agent, and heat-treating the article by raising its temperature progressively in a single stage to a temperature at which devitrification takes place, so as to devitrify the glass to a predominantly crystalline glass-ceramic material.

In the prior art, as mentioned above, it has been customary to produce devitrified glass-ceramics by subjecting the glass first to a nucleation stage, and secondly to a,

devitrification stage of heat-treatment. For example, U.S. Patent No. 2,920,971 (S. D. Stookey) discloses a method in which the glass is first subjected to a temperature, of which the optimum value is 50 C. above the annealing point (Mg point) of the glass, for an optimum time of 0.5 to 2.0 hours. To subject the glass to this temperature immediately after melting, it must of course be cooled from the melting temperature. Where the glass is pressed or drawn in the conventional manner, Stookey heats sample of a glass (identified as Composition 12) in a muflle furnace the temperature of which is increased from room temperature to 870 C., at which the samples are held for half an hour. Samples of another composition (identified as Composition 15) were nucleated by heating them from room temperature to 820 C., at which they were held for two hours.

A further heating step is then employed to cause heterogeneous crystallization of the glass, samples of the first composition (Composition 12) being heated to 1,345 C., which was held for one hour, and samples of the second composition (Composition 15) being raised to 1,250 C., which was held for one hour.

In prior specification No. 2,960,801 (C. B. King et a1.) a nucleation heat-treatment of about 800 C. (775 825 C.) for about one hour is disclosed, followed by a second, or crystallization, stage 1,150-1,200 C. for about four hours.

In prior specification No. 2,960,802 (R. O. Voss), a nucleation heat-treatment of about 800 C. for one hour is disclosed, followed by a second stage of 1,0801,120 C. for about four hours.

In prior specification No. 2,971,853 (S. D. Stookey), the teaching is first to heat the glass between the annealing and softening points of the glass, that is between 500 and 540 C. for the glasses concerned, and secondly to heat the product above the softening point, between 800 and 950 C., to cause further crystallization.

Prior specification No. 3,063,198 (Babcock) deals with the devitrification of a solder glass, rather than of a solid glass product, but nevertheless discloses a sequence of heat-treatments including a nucleating stage at a lower temperature (e.g. 450 C. for one hour) followed by a devitrification stage at a higher temperature (e.g. 525 C. for one hour).

Glasses which may be formed into glass-ceramic arproportion of nucleating agent should total at least percent of the glass. The remaining 10 percent of the glass may be made up of various non-essential constituents some of which take part in the glassy matrix of the ceramic together with residual SiO A1 0 Li O and MgO. All percentages specified in thi specification are percentages by weight. The nature of the non-essential constituents and the amounts by weight which are permissible are as follows in percentages of the glass:

(i) K 0 O-S (ii) ZnO 08.8

The nucleating agent may for example be tungsten oxide, or molybdenum oxide, or both, in an amount such as to be equivalent in the finished product to between 0.5 and 4.0 percent by weight of the glass of molybdenum trioxide or of tungsten trioxide or of both combined; or alternatively the nucleating agent may be a metallic phosphate in an amount such as to give in the finished product a quantity of the phosphate anion corresponding to 0.5 to 3.5 percent by weight of phosphorus pentoxide together with tungsten oxide, molybdenum oxide, vanadium oxide, and/ or titanium oxide in an amount such as to be equivalent in the finished product to between 0.5 and 4.0 percent by weight of the respective metal trioxides either alone or combined in the case of tungsten and molybdenum, to between 0.5 and 2.0 percent by weight of vanadium pentoxide in the case of vanadium and to be tween 0.2 and 1.5 percent by weight of titanium dioxide in the case of titanium.

The following batch materials may be used for the major constituents:

Ground quartz SiO Aluminium oxide A1 0 Aluminium hydroxide Al(OH) The batch materials are thoroughly mixed before melting. As a further constituent of the batch a suitable quantity of the nucleating agent is added which may conveniently be in the form of molybdenum trioxide and/ or tungsten trioxide, in an amount such as to be equivalent in the finished product to between 0.5 and 4.0 percent by weight of molybdenum trioxide or of tungsten trioxide or of both combined.

Alternatively a metallic phosphate may be used as thenucleating agent in combination with the oxide of tungsten, molybdenum, vanadium and/ or titanium in the amounts defined earlier in the specification.

The batch mixture is melted in crucibles at a temperature in the range 1,200 C. to 1,500" C. depending on the composition. The glass is then shaped to produce the desired article by normal glass-working processes, such as casting or pressing.

If it is required to store the articles before devitrification they are annealed at a suitable temperature depending on the glass composition. For the process of controlled devitrification, the temperature of the articles is raised (either from cold, if the articles have been stored, or from the temperature reached by the articles after working) at a rate not exceeding 10 C. per minute and preferably at between 3 C. and 5 C. perminute progressively, in a single stage, to the final crystallization temperature, which varies from approximately 700 C. to 1,250 C. depend ing on the composition. This temperature is maintained for a period of not less than 15 minutes and preferably for two hours, again depending on the composition and during this stage the crystallization occurs and a dense ceramic product containing closely interlocking crystals is ob tained. The articles are then allowed to cool at a rate not exceeding 10 C. per minute, the normal cooling rate of the furnace usually being satisfactory.

TABLE 1-B Composition No.

TABLE Il-A Composition No. Heat-Treatment 1 (a) 1 (b) 1 (c) 2 3 4 5 6 7 8 (a) Temperature, C 1, 000 1, 050 1, 025 1, 000 1, 000 1, 000 1, 000 1, 000 1, 000 1, 000 Hours 2 2 2 5 5 2 2 2 5 5 Pro erties of glas lliodulus of Rupture, p.s.i. X 10- 21 23. 2 24 11. 9 28.1 31. 4 23. 6 10.3 Expansion coeff. per G.X1O- (20-500) 19. 6 20. 6 20, 6 20 6. 9 29. 6 17; 3 3. 5 5. 4 3.8 Main crystal phase in glass-ceramic.

l Beta-eucryptite.

TABLE 11- Composition No. Heat-Treatment 8 (b) 9 10 11 (a) 11 (b) 12 (a) 12 (b) 13 14 (a). 14 (1)) Temperature, C 800 1, 000 1, 000 l, 000 950 1,000 950 1, 000 1, 000 1, 050 mrrs 6 5 5 5 24 5 24 5 2 2 Properties of glass-ceramic:

Modulus of Ruffature, 101-g T22)" 10 17. 6 11. 8 E .nsion coo or 68 0. 6. 2 2.3 9.9 12.0 5.2 17.1 5.4 22. 3 8. 2 19, 0 Main crystal phase in glass-ceramic. Q)

1 Beta-eucryptite.

Specific examples of devitrified glass-ceramic compositions which have been heat-treated in accordance with the method of the present invention are given in Tables 1-A and 1-B.

TABLE 1-A Composition No.

Where one composition has been subjected to different alternative heattreatments, these are designated (a), (b), etc. Compositions numbers 1, 2 and 3 were raised to the final crystallization temperature at the constant rate of 5 C. per minute. Tables IIA and 11-13 also show certain of the physical properties of the resulting glass-ceramics, in particular the modulus of rupture, the thermal expansion COfilClSllt, and the main crystal phase as determined by X-ray diffraction analysis.

The glass-ceramics thus formed were of microcrystalline structure, non-deformed and uncracked, and have, as will be seen'from Tables II-A and ILB, good mechanical strengths and linear thermal expansion coeflicients of less than*30 10-' in the range 20'-500 C.

The main crystal phase of the glass-ceramics for which a main crystal phase is not shown in Table Il-A or 11-13 is a phase varying in a continuous series of solid solutions, between which significant variations cannot be distinguished, from beta-eucryptite to beta-spodumene. Those predominantly beta-eucryptite are associated with the lower coefficients of thermal expansion, and those predominantly beta-spodumene are associated with the higher coetficients of thermal expansion.

We claim:

1. A method of making a glass-ceramic article which includes forming the article in a glass consisting of, as major constituents, silicon dioxide, aluminium oxide, lithium oxide and optionally magnesium oxide within the following ranges in percentages by weight:

sio 6066 A1203 18-20 Li O 5.04.0 MgO 04.0

and further consisting of at least one nucleating agent selected from the group consisting of tungsten oxide, molybdenum oxide, phosphorus pentoxide, vanadium oxide, and titanium oxide, the major constituents together References Cited UNITED STATES PATENTS 2,920,971 l/l960 Stookey 65-33 X 3,063,198 11/1962 Ba-bcock 65-33 X 3,113,877 12/1963 Janakirarna-Rao 6533 OTHER REFERENCES Central Glass Ceramic Research Institute Bulletin with the nucleating agent totalling at least 90 percent of (India), vol. 10, No. 2, 1963, article entitled Nucleation the glass, said method including the step of heat-treating said glass article by raising its temperature substantially directly in a single stage to a value of the temperature at which devitrification takes place, holding said article at said value of the temperature for a sufiicient period to devitrify said glass to a predominantly crystalline glassceramic material, and allowing it to cool to room temperature.

2. A method as claimed in claim 1 wherein the temand Centralled crystallisation of Glass, by R. L. Thakur, pages 51 to 66. Presented at a seminar at the Central Glass and Ceramic Research Institute, Mar. 12, 1962, Calcutto32, India.

S. LEON BASHORE', Acting Primary Examiner.

DONALL H. SYLVESTER, Examiner.

F. W. MIGA, Assistant Examiner. 

1. A METHOD OF MAKING A GLASS-CERAMIC ARTICLE WHICH INCLUDES FORMING THE ARTICLE IN A GLASS CONSISTING OF, AS MAJOR CONSTITUENTS, SILICON DIXOIDE, ALUMINUM OXIDE, LITHIUM OXIDE AND OPTIONALLY MAGNESIUM OXIDE WITHIN THE FOLLOWING RANGES IN PERCENTAGES BY WEIGHT: 